We have been surveying the distribution and comparing the specificity patterns of bacterial xanthine dehydrogenases and related purine oxidizing enzymes. Several of these enzymes have been, or are being, purified and the molecular weights, subunit and prosthetic group contents being determined for comparison with each other and the literature. The specificity pattern of the selected enzymes will be compared in detail through a systematic use of substrates and substrate analog inhibitors. Each of these enzymes display a characteristic pattern of substrate specificity, yet, all the enzymes are capable of oxidizing a wide variety of purine and purine analog substrates. The results with several of the enzymes suggest that the preparations contain several enzyme forms which are in equilibrium and each displaying more limited specificity than the total preparation. We have been manipulating the specificity pattern of several of these preparations by the isolation of mutants resistant to certain of the purine analogs such as allopurinol, purine, and benzimidazole. Recently we have been focusing our attention on the enzymes from two aerobic bacteria, Arthrobacter and Pseudomonas.